8B.5 The LaGrange Tornado during VORTEX2: Single- and Dual-Doppler Analysis of the Tornado

Tuesday, 27 September 2011: 11:45 AM
Urban Room (William Penn Hotel)
Nolan Atkins, Lyndon State College, Lyndonville, VT; and R. M. Wakimoto and J. Wurman

Single- and dual-Doppler radar analyses are combined with cloud photogrammetry of the LaGrange Wyoming tornado on 5 June 2009 during VORTEX2 in an attempt to relate the hook echo, weak-echo hole (WEH), and the intense circulation to the visual characteristics of the tornado. The tornado was not extensive based on a post-storm survey and led to an EF-2 rating. A WEH within the hook echo formed before the appearance of a funnel cloud. The echo pattern through the hook echo on 5 June undergoes a dramatic evolution. Initially, the minimum radar reflectivities are near the surface and the WEH does not suggest a tapered structure as suggested in past studies. Subsequently, higher reflectivities appear near the surface when the funnel cloud makes “contact” with the ground. The WEH is nominally wider than the visible funnel cloud.

A detailed analysis of the double-ring structure within the hook echo is presented. The inner high reflectivity region is believed to be lofted debris. At higher elevation angels, a small WEH appeared within the high reflectivity region owing to centrifuging of debris.

Analysis of the dual-Doppler data suggests that the visual appearance of the funnel does not correlate well with the strength of the radar-detected circulation. Radar-determined tornadogenesis occurred 14 minutes before the time that the funnel was observed to continuously make contact with the ground. Furthermore, as the funnel descended from cloud base to the ground, the low-level vertical vorticity magnitude decreased.

Cyclonic and anticyclonic lowered cloud bases were observed within the hook echo. Vortex lines connected the lowered cloud bases in the form of arches. This observation is consistent with previous studies and suggests that the circulations are formed by tilting baroclinic vorticity by the updraft.

The radius of maximum wind (RMW) was superimposed on the tornado funnel. At or above cloud base, the RMW was observed at the edge of the funnel. Closer to the ground, the observed funnel tapered to smaller diameters whereas the RMW did not. As a result, the RMW was observed well beyond the edge of the funnel near the ground.

Finally, the evolution of angular momentum within the hook echo was examined. Angular momentum exhibited a maximum below cloud base at large radii from the vortex center. The largest inward radial transport of angular momentum occurred before the funnel was observed to make contact with the ground. Later, as the funnel widened with time, the radial gradient and inward transport of angular momentum weakened.

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